Euploid Monosomic (A) Ditelosomic (AL) Nullisomic

A
Telomere
Centromere
B
D
Euploid
A wheat cultivar such as 'Chinese Spring' (CS) has 21 pairs of these
'complete' chromosomes organised as seven groups of the three
homoeologous genomes (A, B, D). No chromosome is completely
metacentric, i.e. the arms are not identical in length when observed during
mitotic metaphase under a high-power microscope. Such obersvations many
decades ago led to the designation of short and long arms
that are still use use today.
When self-pollinated, this stock will breed true, viz. both male and
female gametes of all members of the progeny will each contain 21
chromosomes.The subsequent zygote in all will contain 21 pairs of
chromosomes. N.B. Remember that aneuploidy can occur at a rate of
1x10-3, so even the CS euploid stock must be cytologically verified at
each regeneration!
Telomere
Monosomic (A)
A monosomic aneuploid stock has 20 pairs of chromosomes plus only one
of a particular chromosome. When self-pollinated, the univalent can be
present or absent in one or both of the male and female gametes, leading to
a segregating progeny containing nullisomic, monosomic or euploid plants.
Some nullisomic configurations are rarely seen because the chromosome
concerned affects meiotic division, juvenile development, etc.
Ditelosomic (AL)
A chromosome with its centromere at one or other end is said to be telocentric
and is referred to as a telosome. It is the result of misdivision in one of the
gametes where a chromosome divides transversely at the centromere instead
of longitudinally. The two chromosome arms move to opposite poles and end up
in different daughter cells. The F1 progeny will thus possess 20 pairs of complete
chromosomes and a heteromorphic pair (composed of the complete and telosomic
homologues. The F2 progeny will contain some monotelosomic plants, which
when selfed will yield ditelosomics in their progenies, i.e.20 complete pairs and
a telosomic pair. This aneuploid stock is known as a ditlelosomic (typically
abbreviated to ditelo), and is labelled according to which half chromosome is
present, e.g. 1AS, 1AL, 2AS, etc.. The process leading to the creation of a ditelo
is depicted in Appendix I.
NullisomicTetrasomic (A-D)
Ernie Sears, a pioneer of aneuploid study in wheat, was able to
select plants from F1 progeny which had one dose of a particular
chromosome coupled with three doses of one of its homoeologous
chromosomes. When these plants are selfed, the progenies will
carry the odd plant which has lost the former but which possesses
two pairs of the latter. This process is depicted in Appendix II.
The compensating effects amongst homoeologues in bread wheat
are generally very good. However, the infertility of some nullisomictetrasomic stocks means they have to be maintained in a
monosomic-tetrasomic state. Parental types have to be selected
once more to regenerate the stock, but marker studies can utilize
the DNA extracted from juvenile nullisomic-tetrasomic plants.
Similarly, such nulli-tetra plants can also be used as female parents
in chromosome substitution exercises, etc.
A
B
D
Double Ditelosomic (A)
Sometimes when misdivision creates two half chromosomes or telosomes
(see explanation of ditelosomics), both can move to the same pole. Although
the odds are lower, some of the individuals in the progeny will have a pair of
both arms or telosomes, termed a double ditelosomic stock.
However, Prof. Ernie Sears couldn't wait for such random events to occur
and created his double ditelosomic series by crossing the single ditelosomic
with its counterpart and then selfing the F1, e.g.
ditelosomic 3AS x ditleosomc 3AL, then self > double ditelosomic 3A.
Tetrasomic (A)
Sometimes at meiosis, anomalies occur where both chromosomes
of a dividing pair move to the same pole and create a gamete with a
disomic pair (remember gametes have the haploid number of
chromosomes). The resultant zygote involving such a gamete
thus has three of the same chromosome, and the plant that grows
is known as a trisomic. Selfing the plant can produce those having
four, i.e. 2 pairs of the same chromosome, or a tetrasomic.
At meiosis, these four identical chromosomes often join together to
form a quadrivalent rather than two bivalents.
Di-isosomic (AL)
Telocentric chromosomes are derived from the chromosome misdivision during
meiosis when the plane of initial breakage is transverse instead of longitudinal
(see the ditelosomic above for full explanation, and also Appendix III).
On rare occasions, the chromatid partners are aligned more at right angles to
each other rather than along their length when the centromeric break occurs.
Two identical arms still joined at the centromere can thus be dragged towards
a pole, forming a chromosome with identical arms, i.e. an isochromosome.
As far as genetic content goes, this stock is the same as a ditelo. The one
advantage is when the isochromosome is able to transmit through both male and
female gametes to create a di-isosomic, which then has four doses of the same
chromosome arm.
Nullisomic (A)
See the explanation for the monosomic aneuploid above. A nullisomic arises
during selfing of a monosomic stock and it lacks a pair of chromosomes.
Although bread wheat is an allopolyploid (derived from three different parents),
fortunately all three exhibit high levels of homoeology (they have many similar
genes which are also similarly located on their chromosomes). However, not all
nullisomics can exist because sometimes the compensating effects of these
homoeologous chromosomes are insufficient to enable the plant to produce seed.
Appendix I: Origin of Ditelos
1
2
upper
gamete
3
Pairing of
upper gamete
with normal
gamete
4
Small proportion
of selfed progeny
Appendix II: Origin of Nulli-Tetras
Monosomic (A)
Tetrasomic (D)
x
Possible
Gametes
x
Select
Self
Possible
Gametes
x
Select
Nullisomic-Tetrasomic (A-D)
Appendix III: Origin of Di-isosomics
3 - isosomic (A)
1
Pairing of
upper gamete
with normal
gamete
upper
gamete
2
Small proportion
of selfed progeny
4 - Di-isosomic (A)